1. Field of the Invention
The present invention relates to biaxially oriented films formed of a film forming composition comprising a polyoxymethylene polymer in which the major constituent of the main chains of said polymer each consists substantially of repeating units of oxymethylene group--CH.sub.2 --O--. More particularly, the present invention relates to the above described biaxially oriented polyoxymethylene films excellent in surface smoothness.
2. Statement of the Related Art
It is generally known that polyoxymethylene has a very remarkable tendency to crystallize. While polyoxymethylene films are formed, for instance, by melt compression or melt extrusion followed by quenching, the films thus obtained are opaque or translucent and are additionally characterized with inferior mechanical properties. It is said that this is because crystallized polyoxymethylene forms highly-grown spherulites. Accordingly, attempts have been made to improve the clarity and mechanical properties of polyoxymethylene films by stretching these films biaxially.
Japanese Patent Application Kokai (Laid-Open) Nos. 252135/86, 286115/86, and 013318/87 (corresponding to Japanese Patent Application Nos. 92246/85, 127132/85, and 151349/85, respectively), filed previously by the present inventors, disclose that a highly biaxially oriented crystalline polyoxymethylene film has a high tensile elastic modulus and a superior dimensional stability, and further reveal techniques for the industrial production of such films, whereby the production of biaxially oriented polyoxymethylene films having excellent properties has become possible.
These biaxially oriented polyoxymethylene films are by far superior in strength, dimensional stability, clarity, etc. However, the surface smoothness of these films is not on a sufficient level as compared with those of other common polymer films, e.g. polyester films. The surface smoothness may be a very important factor depending upon the application of the film. For example, the improvement of magnetic tapes has rapidly advanced rapidly in recent years and this has been accompanied, in particular, by severe surface quality requirements for the base films. In order to achieve high performance in magnetic video recording, the tape function of electric energy-magnetic energy interconversion must be enhanced and for this reason the surface of the base film is required to be smoothed. To be suitable for magnetic recording applications, it is required to control the surface roughness of the base film to a centerline-average roughness Ra of 0.1.mu. or less.